The present invention pertains generally to material bonding, and more particularly, to a method and apparatus for using hot plasma gas to simultaneously heat, clean, and activate the bonding surfaces of bondable solid sheets before and during bonding of the sheets together.
Various techniques exist for bonding materials together. For materials comprising metals, bonding techniques include welding, brazing, soldering, and more recently, direct contact and diffusion bonding. For materials comprising non-metal solids, bonding techniques typically include using some type of adhesive such as an epoxy.
Welding is a metal joining process wherein coalescence is produced by heating the metal to suitable temperatures, with or without the application of pressure and with or without the use of filler metals. Coalescence is a growing together into one body. A weld is the junction of sheets or the edges of sheets which are to be joined or have been joined by melting and refreezing at the contact interfaces of sheets themselves. Filler metal is the material to be added in making a welded, brazed, or soldered joint. Base metal is the material to be welded, soldered, or cut.
The term arc welding applies to a large and varied group of processes that use an electric arc as the source of heat to melt and join metals together as the molten metal re-freezes. In arc welding processes, the joining of metals, or weld, is produced by the extreme heat of an electric arc drawn between an electrode and the work-piece, or between two electrodes. The formation of a joint between metals being arc welded may or may not require the use of pressure or filler metal. The arc is struck between the workpiece and an electrode that is mechanically or manually moved along the joint, or that remains stationary while the workpiece is moved underneath it. The electrode is either a consumable wire rod or a nonconsumable carbon or tungsten rod which carries the current and sustains the electric arc between its tip and the workpiece. When a nonconsumable electrode is used, a separate rod or wire can supply filler material, if needed. A consumable electrode is specially prepared so that it not only conducts the current and sustains the arc, but also melts and supplies filler metal to the joint, and may produce a slag covering as well.
Gas welding processes are a group of welding processes in which a weld is made by heating with a gas flame. Pressure and/or filler metal may or may not be used. Also referred to as oxyfuel gas welding, the term gas welding is used to describe any welding process that uses a fuel gas combined with oxygen, or in rare cases, with air (20% Oxygen), to produce a flame having sufficient energy to melt the base metal. The fuel gas and oxygen are mixed in the proper proportions in a chamber, which is generally a part of the welding tip assembly. The torch is designed to give the welder complete control of the welding flare, allowing the welder to regulate the melting of the base metal and the filler metal. The molten metal from the work-piece edges and the filler metal intermix in a common molten pool and join upon cooling to form one continuous piece.
Brazing and soldering are welding processes in which materials are joined by heating to a suitable temperature and by using a filler metal with a melting point below that of the base metal. The filler metal is distributed to the closely fitted surfaces of the joint by capillary action.
Resistance welding consists of a group of processes in which the heat for welding is generated by the resistance to the electrical current flow through the parts being joined, using pressure. It is commonly used to weld two overlapping sheets or sheets which may have different thicknesses. A pair of low resistance electrodes conducts electrical current through the sheets, forming a weld. The Key aspect of the resistance weld is that most of the resistance in the welding circuit is in the contact resistance where the metals are pressed together, so that most of the (I{circumflex over ( )}2*R) heat is formed at the surfaces that are to be welded together.
The properties of a welded joint depend partly on the correct preparation of the edges being welded. Cleanliness is of key concern. All mill scale, rust, oxides, and other impurities must be removed from the joint edges or surfaces to prevent their inclusion in the weld metal. The edges should be prepared to permit fusion without excessive melting. Care must be taken to keep heat loss due to radiation into the base metal from the weld to a minimum. A properly prepared joint will keep both expansion on heating and contraction on cooling to a minimum.
Diffusion bonding is a method of joining metallic or non-metallic materials. This bonding technique is based on the atomic diffusion of elements at the joining interface. Diffusion process actually is the transport of mass in form of atom movement or diffusion through the lattice of a crystalline solid. Diffusion of atoms proceeds by many mechanisms, such as exchange of places between adjacent atoms, motion of interstitial atoms or motion of vacancies in a crystalline lattice structure. The latest is the preferable mechanism due to low activation energy required for atom movement. Vacancy is referred to an unoccupied site in a lattice structure. Both diffusion and direct contact bonding are preferable bonding methods. Direct Contact/Fusion bonding occurs when the surface atoms are brought within atomic distances with the application of pressure and heat. Any surface contamination will inhibit bonding.
Diffusion bonding involves diffusion of atoms via a thermodynamic process where temperature and diffusibility of the material are considerable parameters. In general, the diffusion rate, in term of diffusion coefficient D, is defined as D=Doxe2x88x92Q/RT, where Do is the frequency factor depending on the type of lattice and the oscillation frequency of the diffusing atom. Q is the activation energy, R is the gas constant and T is the temperature in Kelvin.
The activation energy for atomic diffusion at the surface, interface and grain boundaries is relatively low compared to the bulk diffusion due to a looser bond of the atoms and higher oscillation frequency of the diffusing atom. This enhances the atomic diffusion, and thus eases the diffusion bonding of two metal pieces assuming that a perfect interface contact exists.
Since diffusion bonding is driven by the diffusion of atoms, diffusion bonding process can be used to bond dissimilar materials that are difficult to weld, such as, steel and copper alloys. When bonding metals together, direct contact/fusion and diffusion bonding causes micro-deformation of surface features, due to the pressure and temperature applied, which leads to sufficient contact on an atomic scale to cause the materials to bond. However, before the materials can be joined they undergo an extensive preparatory treatment.
The interface contact can be optimized by a treatment of the surface to be bonded through a number of processes, such as mechanical machining and polishing, etching, cleaning, coating, and material creeping under high temperature and loading.
All of the above-mentioned bonding methods are problematic due to the inability to prevent contaminants from adhering to the bond site surfaces. Surface contamination causes poor bond adhesion, resulting in less robust bonds and therefore less reliable interfaces. Accordingly, much effort has gone into cleansing the surfaces of metals prior to being bonded.
Low temperature plasmas of various ionized gases can be used to reactively etch/ash organic materials found on the surface of materials. In this procedure, typically the material is placed in an RF cavity with a flowing reactive gas. The nature of the gas selected is chosen based upon the desired effect. Oxygen or argon is generally used, however, specific gases (CF4) may be used to tailor the reaction for the desired effect. By removing surface contamination, plasma cleaning increases the bonding or adhesive properties of the bondsite surface.
A plasma is a collection of positive, negative, and neutral particles wherein typically the density of the negatively charged particles is equal to the density of the positively charged particles. When an energetic electron strikes a neutral gas molecule, it can cause dissociation and form free radicals and ions. The free radicals cause chemical reactions for destroying contaminants. For example, with oxygen, the dissociation process produces the free radical atomic oxygen (O). This reactive species has enough energy to break a carbon-carbon bond. When the plasma gas is a mixture of hydrogen and argon, for example, the free radical atomic oxygen (O) can therefore combine with the hydrogen (H2) into water (H2O).
Prior art plasma cleaning involves placing the workpieces to be bonded into a plasma chamber, creating a vacuum within the chamber by pumping out all the air from the chamber, introducing a gas or gaseous mixture into the chamber, and energizing the gas in the chamber to produce the plasma. In the presence of the plasma, organic contaminants on the bondsite surfaces are converted to carbon monoxide, carbon dioxide, water vapor, and/or other gasses, which are pulled out of the plasma chamber by a vacuum pump. After a predetermined amount of time, the gas flow and energy are shut off, and the chamber is then purged with a nonreactive gas, such as nitrogen, to remove all traces of volatile compounds. Finally, the chamber is returned to atmospheric pressure. A cleaning cycle usually lasts from between 30 seconds to 15 minutes or more and is largely a function of the workpiece loading in the plasma chamber.
An important application of plasma cleaning is the removal of aluminum oxide (sapphire) from the surface of aluminum to allow diffusion bonding. Oxygen has such a strong affinity for aluminum that in a normal atmosphere, pure aluminum becomes oxidized in seconds, forming a few atomic layers of aluminum oxide on the surface, preventing aluminum-to-aluminum bonding. Plasma bathing of the metal sheets reduces the surface aluminum oxide back to the original elemental aluminum, allowing bonding to take place.
Current metal diffusion methodologies have been unsuccessful for creating aluminum bonds, because of the difficulty of removing aluminum oxide chemically and because of the speed with which aluminum oxidizes. Further, the heating required to make the metal ductile enough to bond accelerates oxidation, which prevents bonding. With current , sequential, methods for cleaning, heating, and pressing bond surfaces, reoxidation prevents diffusion bonding of aluminum.
The current plasma cleaning methodology is problematic. First, immediately upon emerging from the plasma cleaning chamber, the cleaned workpiece(s) are re-exposed to surface contamination due to the organic particles in the air. In addition, the separate plasma cleaning chamber and equipment, coupled with the significant amount of time required to set up and execute the cleaning process, is quite costly.
It is therefore an object of the invention to provide a method for simultaneously heating, cleaning, and bonding materials together.
The present invention is a novel plasma enhanced bonding method and apparatus. The invention effectively generates a plasma bath, or xe2x80x9cbubblexe2x80x9d, around the bonding surfaces of solid sheets to be bonded prior to and during the bonding action to thereby simultaneously heat, clean, and activate the surfaces of the materials that are to be bonded together, while the bonding is taking place.
In accordance with the invention, a pair of sheets to be bonded are positioned within close but non-contacting proximity to, and preferably in alignment with, one another. In a preferred embodiment, the sheets are fed by a sheet feeder between two compression rollers through which the sheets are compressed as they feed between the rollers. Prior to bonding the sheets together at their respective bonding surfaces, hot plasma is applied in a pressurized directional modulated flow at and between the bonding surfaces of the sheets to be bonded together to create a dynamic plasma cleaning chamber bubble. While continuing to apply the modulated plasma flow, the sheets are compressed by the rollers such that the bonding surfaces are brought into contact within atomic distances of one another at their respective bonding surfaces. The mere contact within atomic distances of the bonding surfaces of each sheet with one another while within the dynamic plasma cleaning chamber bubble allows contact bonding; however, diffusion bonding may be achieved by slowing the speed of the compression rollers sufficiently to apply pressure to the sheets therebetween over an appropriate amount of time. Furthermore, the rollers may optionally be heated to provide additional activation energy to achieve diffusion bonding of the sheets together. As the sheets exit from the compression rollers, the pressure is removed, resulting in a robust sheet-to-sheet bond.
Preferably, the plasma cleans and activates the surface while heating the bonding surfaces to allow direct contact bonding to take place as the clean metal surfaces are pressed together such that at least some of the surface atoms are brought within atomic distances of one another, and without melting the bonding surfaces. This eliminates the need for an additional heating mechanism for heating the bonding surfaces.
In a particular application, the inventive process makes it possible to diffusion bond aluminum sheets because the cleaning steps are performed prior to and simultaneously with the bonding. This does not allow recontamination between the preparation of the surfaces and the bonding itself.
Accordingly, the present invention advantageously provides a novel technique that cleans the surfaces to be bonded as they are bonded to eliminate any contaminants from being introduced into the final bond, thereby improving the bond adhesion properties. The invention not only allows differing metals to be bonded together, but may also allow materials to be bonded together that prior to the present invention were difficult to bond due to contaminant compounds formed at the bond site, which resulted in weaker bonds. In addition to the above-named advantages, the temperature of the plasma/hot gas is high enough that the press need not include a heater. The invention essentially provides xe2x80x9clocalizedxe2x80x9d heating directed only at the bonding surfaces, thereby reducing the risk of damage to the sheets to be bonded as well as to any components attached to the sheets. The invention also makes possible the bonding of non-metallic materials such as sapphire, silicon, ceramic, and quartz materials in a similar manner. The direct contact bonding method only requires that the surface finish, temperature, and pressure are adequate for enough of the contact surface to be brought close enough together for the atoms from one surface to bond the atoms in the opposite surface while a modulated stream of intense plasma cleans, heats, and activates those surfaces as the contact happens.